1. Field of the Invention
The present invention relates to an apparatus for manufacturing silicon single crystals by the Chochralski method.
2. Description of the Prior Art
In the field of LSIs, there has been a demand for yearly increase in the diameter of silicon single crystals. At present, crystals of 6 inches in diameter are used for the most modern devices. It has been said that in future crystals of 10 inches or over in diameter, e.g., crystals of 12 inches in diameter will be required.
The Chochralski method (CZ method) involves two concepts with respect to the crystal manufacturing method, that is, the first concept is a method in which a crucible is rotated and the second concept is a method in which a crucible is not rotated. Presently, all of CZ crystals used for LSIs are manufactured by a method of the type in which the crucible is rotated oppositely to the rotation of a crystal. Furthermore, the crucible is heated by an electric resistance heater mainly surrounding the side of the crucible. Despite a variety of attempts, silicon single crystals of over 5 inches in diameter have never been produced up to date and will not be produced in the future by the methods of non-rotation of the crucible or by using other heating processes than the above-mentioned heating process. The reason is that it is impossible to obtain a completely concentric temperature distribution with respect to a growing crystal without rotating the crucible or by using any heating process other than the previously mentioned heating process, such as by electromagnetic induction heating or by electric resistance heating applied to the bottom of the crucible. The growing of such a crystal is extremely susceptible to temperatures.
In the CZ method of the type which rotates the crucible (hereinafter referred to as the ordinary CZ method), the rotation of the crucible and the electric resistance side heating cause a strong convection of the molten silicon and the melt is agitated satisfactorily. Thus, the method is desirable for the growth of large-diameter single crystals. In other words, it is possible to obtain a melt surface temperature distribution which is uniform and which is completely concentric with respect to a crystal. Therefore, the present invention is based on the ordinary CZ method. As mentioned previously, there is a considerable difference in the flow of molten silicon between the ordinary CZ method and the other CZ method. This difference results in considerable difference in crystal growth condition between the two. Consequently, the two methods differ considerably with respect to the functions of the component parts within the furnace. The two methods differ entirely in terms of their concepts to the growth of crystals.
In the case of the ordinary CZ method, the amount of the melt in the crucible decreases with the growth of a crystal. Thus, as the crystal grows, the dopant concentration increases and the oxygen concentration decreases in the crystal. In other words, the properties of the crystal vary along its growth direction. This problem must be solved in view of the fact that paralleling the trend toward increasing the density of LSIs, the quality required for silicon single crystals has become increasingly critical from year to year.
As a means of solving this problem, there is known a prior method in which the interior of a quartz crucible according to the ordinary CZ method is divided by a cylindrical quartz partition formed therethrough with a hole or holes so that the molten silicon can pass through whereby a cylindrical silicon single crystal is grown on the inner side of the partition while feeding silicon starting material to the outer side of the partition member (e.g., Patent Publication No. 40-10184, P1, L20-L35). As pointed out in Japanese Laid-Open Patent No. 62-241889 (P2, L12-L16), the major problem of this method is that a solidification 30 of molten silicon tends to occur starting at the partition member on the inner side thereof (see FIG. 7). This is caused by the following fact. As will be seen from the use of quartz for optical fiber, the quartz partition member efficiently transmits heat by radiation. The heat in the molten silicon is transmitted as light upwardly through the partition member and the heat is dissipated from the portion of the partition member which is exposed from the molten silicon surface.
Therefore, the melt temperature is decreased considerably in the vicinity of the partition member. Also, in accordance with the ordinary CZ method, the surface temperature of the melt is not only uniform but also just above the solidification temperature of the melt due to vigorous agitation of the melt. With these facts coupled together, the melt surface in contact with the partition member is in such condition as to have a very great tendency to cause the solidification 30. In order to overcome this deficiency, Japanese Laid-Open Patent No. 62-241889 proposes a method which uses no partition member. However, this method has not been put in practical use since the material melting section is limited and thus the material melting capacity is extremely low.
Where a single crystal is pulled while continuously and directly feeding granular silicon into the crucible in accordance with the previously mentioned conventional techniques, the following problems are encountered.
(1) Where the crucible is divided into a granular silicon melting section and a single crystal pull section, solidification of the melt tends to occur from the inner side of the partition member. Then, once the solidification occurs, it continues to grow thus impeding the growth of a sound single crystal.
(2) With a view to solving this problem, if the partition member and the material melting section are covered by a heat keeping plate ensuring a large amount of heat transfer within the plate surface, a crystal to be pulled itself is cooled by the heat plate. As a result, the temperature gradient in the crystal deviates from the single crystal growth enabling temperature region, thus impeding the growth of a sound crystal.
(3) During the pulling of a silicon single crystal, the temperature of the melt is considerably close to the melting point of silicon. In this condition, if granular silicon which temperature close to the room temperature is fed continuously, the granular silicon is not melted completely and thus floats in its solid condition to the surface of the melt, thereby causing the melt to solidify and grow with the granular silicon as a nucleus.